Influence of uniform magnetic field on physicochemical properties of freeze-thawed avocado puree
External fields are reported to have the potential to improve the quality of food after freeze-thawing. This study investigated the effect of an oscillating magnetic field with high uniformity and multi-directions at 4 mT and 50 Hz on the freezing of avocado puree. Samples of avocado puree were placed in the central zone of three pairs of orthometric Helmholtz coils where a highly homogeneous degree (99% homogeneity) magnetic field could be guaranteed. For comparison, avocado puree was frozen without the magnetic field. According to the freezing curve, under the magnetic field, the freezing point of avocado puree was decreased from −1.2 to −6 °C, which indicated that the temperature zone for maximum ice crystal formation was reduced. The uniform magnetic field had a positive effect on the avocado puree. The pH value of the samples after the magnetic field-assisted freezing was higher than that of the control. With the increase in the magnetic field direction, ΔE gradually decreased. In addition, the antioxidant activity of the puree significantly decreased, and the diphenylpicrylhydrazyl (DPPH) radical scavenging activity decreased from 69.18% to 56.22%, whereas the ferric reducing antioxidant power (FRAP) value decreased from 18.5% to 14.75%. However, the three-direction magnetic field-assisted freezing could better maintain the antioxidant activities of avocado puree. Moreover, the interaction of ferrous gluconate and multi-directional magnetic field could exert an improvement on the quality of freeze-thawed avocado puree, owing to the small molecular current.